The invention relates to a brake control arrangement for controlling braking of a composite vehicle including a tractor and at least one trailer, and more particularly to such a brake control arrangement having an electronic brake control and a mechanical brake control.
For a truck or semitrailer tractor, the brake pressure required for braking is fed to the trailer (or semitrailer) by means of a compressed air line or the like. In an ideal case, said brake pressure should be adjusted or regulated such that the trailer is decelerated at exactly the same braking rate as its tractor. A braking behavior of the trailer and tractor which is matched in such a way has the considerable advantage that the braking process may be controlled with great precision and neither the trailer nor the tractor may jack-knife. It is permissible, however, to select the braking deceleration of the trailer to be slightly greater than that of the tractor, since, in such a case, on one hand, the braking process is still controllable and, on the other hand, this treats the maintenance intensive brakes of the tractor with care.
Since, in the course of time, the braking power of the brakes of the trailer changes to a different extent than those of the tractor, the desirable relationship, explained above, of the braking power of trailer and tractor can be guaranteed only with maintenance at relatively short service intervals. Furthermore, it must be taken into consideration that the weight of the trailer changes as a function of the respective load, such that a precise adjustment is possible in any case only in the presence of a so-called automatic load-dependent brake device ("ALB"). But this is the case only with modern trailers, where an ALB-device is present in the trailer, which modifies, as a function of the trailer weight, the brake pressure (brake requirement) fed from the tractor and feeds said modified brake pressure to the brakes of the trailer.
In order to obtain a better braking effect, even in a composite vehicle, the trailer of which is not yet equipped with the ALB function, a so-called "coupling force regulator" has been proposed, for which the force appearing at the coupling between tractor and trailer is measured and the brake pressure for the trailer is regulated as a function of said force such that the coupling force acting in the lengthwise direction of the traction link is as limited as possible. In the braking process, this signifies that the amounts of braking deceleration of the tractor and trailer are of equal magnitude.
Examples of such a regulation of the coupling force are shown in DE 42 20 991 A1, U.S. Pat. No. 5,002,343 or DE 40 35 805 C1.
Since the use of sensors to measure the coupling force is problematic in the rough operation of trucks (because of the frequent failure of sensors) and since said sensors are also relatively expensive, it was proposed, in the not previously published German Patent Application P 44 12 430, and the likewise not previously published German Patent Application P 44 46 358.8, to abandon such coupling force sensors and to detect the slope and weight of the trailer by means of the use of an energy equation with the use of measurable variables present in the tractor, such as vehicle velocity and driving energy.
Likewise, it is known from EP 0 621 161 A1, how the force acting between the tractor and the trailer may be detected and used in order to regulate the brake pressure of the trailer.
Also becoming more common in trucks are electric, or electronic, brake controls (EBR), for which an electronic control device prepares a pneumatic or hydraulic brake pressure for the brakes of the tractor based on the brake demand of the driver and a multitude of additional measured and/or stored parameters. For safety reasons, it is specified, as a matter of principle, that in case of error in the EBR, a conventional mechanical brake control arrangement must be present in addition, which, in a very conventional way, produces a pneumatic or hydraulic brake pressure. For said brake system, which is available on the market, a trailer control valve is provided which is arranged in the tractor, to which are fed the brake pressures of the electronic brake device (EBR) and of the conventional mechanical brake device. The trailer control valve is constructed such that, in each case, it directs the maximum pressure at both of its inputs through to the brakes of the trailer. The trailer control valve is consequently a maximum value selection valve.
With a modern trailer, which already features automatic load-braking (ALE), the brake pressure routed by the trailer control valve to the trailer is modified as a function of the trailer load.
In some countries, the use of automatic load gradual braking (ALB) in a trailer or semitrailer is not common, nor is it expected to be in the future. This signifies a considerable forfeiture of stability. Even if such a vehicle is equipped with an antilock system (ABS), its function will worsen since it must always regulate from a pressure level which is too high. Further, the wear on the tires is greater, since, for all braking during operation of a partially loaded or empty semitrailer or trailer, the brake pressure for the trailer is too high and, as a result, the antilock system is employed, resulting in the occurrence of slippage values of approximately 15% instead of only approximately 1%, as is common for a load-dependent brake.